Several methods have been developed for the introduction and expression of genes in plant and animal cells and particularly plant cells. These methods have a number of limitations, especially considering the complexity of the breeding process required for the introgression of more than a few genes into elite lines and obtaining stable, predictable expression of the individual genes. The current methods for plant transformation are limited to the introduction of small segments of DNA, generally sufficient for the expression of two to three genes. With the rapid increase in rate of sequencing and the discovery of new genes for modifying agronomic traits and for directing plants to synthesize material from entirely new pathways, this limitation will be severely limiting. In addition, the current method of randomly introducing genes into the genome of the recipient plant leads to extensive linkage drag, potential for disruption of important genes and confounding the production of elite lines.
The artificial chromosome is a linear piece of DNA that contains all the necessary elements for stable replication and segregation. Artificial chromosomes have been described for yeast (Burke et al., Science 236:806-812, (1987)), bacteria (O'Connor et al., Science 244(4910):1307-1312 (1989), Shizuya et al., Proc. Natl. Acad. Sci. USA 89(18):8794-8797 (1992), Hosoda et al., Nucleic Acids Res. 18(13):3863-3869 (1990)), and more recently for animals (Harrington et al., Nature Genetics 15:345-355 (1998); Grimes and Cooke, Human Molecular Genetics 7(10):1635-1640 (1998); and Ikeno et al., Nature Biotechnology 16:431-439 (1998)). In these cases, the chromosomes were produced by identifying the required elements and then manipulating them to build a chromosome, or via in vivo and in vitro manipulations involving isolation of one or more chromosomal elements.
U.S. Pat. No. 5,270,201 describes telomeric sequences from Arabidopsis and use of those sequences to construct a plant artificial chromosome. The patent disclosure relates to a recombinant DNA molecule that contains the telomere and optionally the centromere of a higher eukaryote. To provide a functional artificial chromosome in accordance with the teachings of the patent, the functional elements of a chromosome must be assembled and transformed into a plant cell. The element exemplified in the patent, the telomere, is the simplest one of the necessary pieces. Presently however, a plant centromere is known to be a highly complex structure of at least 360,000 base pairs. More recently, PCT application (WO 98/55637) describes the identification and cloning of functional plant centromeres based on Arabidopsis. 
Hence, there is a need in the agricultural biotechnology arts for methods of producing plant artificial chromosomes that entail less complex genetic manipulation and assembly of individual chromosomal elements.